1. Field of the Invention
The present invention relates generally to a method for determining the condition of motor oil, and more particularly to a method of monitoring motor oil during diesel engine operation in implementing an economically efficient oil change regimen.
2. Description of the Prior Art
Truck fleet operation is a highly competitive and fleet operators are highly conscious of balancing preventive maintenance costs and fleet operational readiness with repair and replacement costs. While cost effective fleet management and readiness requirements demand the practice of preventive maintenance, some of these practices are based, for lack of individual vehicle information, on potentially overly conservative regimens derived from broad statistical studies. An example of such a practice is manufacturers"" recommendations relating to changes of motor oil. A change of motor oil costs money and removes a vehicle from service. While frequent changes of motor oil doubtless extend the service life of a vehicle and are, up to a point, economically advisable, they can become wasteful and uneconomic if excessively frequent.
Changes of motor oil are necessitated by the fact that motor oil loses its lubricating properties with use. With loss of adequate lubrication an engine is exposed to wear and damage. The degree, and character, of motor oil degradation is related to a number of factors, including temperature cycling of the lubricant which relates to oxidation of the oil, and the possibility of the addition of foreign material to the oil (e.g. soot). Manufacturers"" recommended oil change schedules are typically based on a conservatively short estimate of the useful life of the oil.
U.S. Pat. No. 5,987,976 reports the practice of modifying estimates of engine oil useful life by taking into account the conditions under which an engine operates. The ""976 patent asserts that a short coming of such a method is that it does not indicate the condition of the motor oil at any given moment. Put another way, the method is a refinement of statistical methodology and is handicapped by the use of operator impressions of xe2x80x9coperating conditionsxe2x80x9d (e.g. vehicle loads, prevailing air temperature, etc.), and the lack of quantifiable inputs relating to specific trucks.
In diesel engines, engine oil condition is primarily related to soot accumulation and molecular degradation due to shear and exposure to high temperature. Soot is a by-product of incomplete combustion of hydro-carbon fuel, which can result from a number of factors, such as use of low volatility diesel fuel blends at low ambient temperatures and operation of an engine at a disadvantageous point on the engine torque curve. In a simplified sense, high engine loads over extended periods of time result in cylinder blow-by which adds soot to the oil and produces localized high temperatures in the oil leading to molecular degradation. Soot moves from the cylinder to the engine oil as a result of cylinder blow-by or by adhesion to the cylinder walls from which it is swept up by the piston rings into the oil. Soot becomes an issue in engine oil when the amount of soot overwhelms dispersants in the oil and begins to agglomerate into particles of sufficient size to damage the engine. Soot intrusion to the engine oil may be estimated as a function of Brake Mean Effective Pressure (BMEP), which serves as a proxy for engine load, and engine revolutions (n). C0 is a proportionality constant.
SOOT=C0∫BMEP dn for n=0 to n=Mxe2x80x83xe2x80x83(1) 
Engine oil molecules are subject to shear due to the mechanical action of tappets, bearings and the oil pump. Shear may also depend upon the type of fuel injector used in the engine. Some fuel injections employ high pressure engine oil, an additional shear contribution source. For a particular engine design, shear is a function of total revolutions of the engine, not vehicle mileage, since the last oil change. K0 is a proportionality constant.
SHEAR=K0∫dn for n=0 to n=Mxe2x80x83xe2x80x83(2) 
Other factors are known to contribute to early oil wear, particularly if engine duty cycles are of short duration. Water can contaminate engine oil from air entering through the oil filler nozzle or from the engine cooling system. If an engine does reach or maintain a minimum threshold operating temperature, water which has contaminated the oil will not be forced by evaporation from the oil. While small amounts of water are not harmful, water combines with sulfur compound combustion by-products to form highly corrosive acids in the engine oil, particularly H2SO4. H2SO4 can overwhelm basic additives to the engine oil and damage an engine. While it is technically inappropriate to refer to engine oil as having a ph factor, water dispersed in the engine oil has a ph factor. Engine oil additives are designed to give water contaminants in the engine an elevated (i.e. basic) ph to neutralize low levels of H2SO4 dissolved in the water. Excessive intrusion of H2SO4 overwhelms the additives.
Another possible engine contaminant is ethylene glycol, which can escape from the engine coolant system. Fuel contamination is also a possibility. Both of these contaminants reduce the lubricity of engine oil and may indicate engine damage.
What is needed is a system implementing a model of engine oil deterioration the variables of which can be effectively measured during engine operation. Preferably the variables can be obtained with no, or a minimal number of, additives sensors.
According to the invention there is provided a method for generating indicators relating to the condition of lubricating oil in an internal combustion engine. A set of variables relating to operation of the internal combustion engine serve as proxy variables for brake mean effective pressure developed by the internal combustion engine. Operation of an engine is monitored to develop values for the proxy variables. Soot being added to the lubricating oil from the developed values and accumulated to provide an estimate of total soot in the lubricating oil. Lubricating oil temperature is periodically monitored and a history of the lubricating oil temperature measurements is kept to allow generation therefrom of an estimate of shear and oxidation of the lubricating oil. Accumulated estimate of soot is kept as a distance remaining until the lubricating oil becomes unsuitable for continued use in the internal combustion engine. Similarly, estimated shear is expressed as a distance until the lubricating oil is to be replaced. Additional factors may be monitored and similary used to determine the distance until an oil change is required, including the dielectric coefficient of the lubricating oil, temperature history of the oil and total cylinder firing events.
Additional effects, features and advantages will be apparent in the written description that follows.